The invention relates to an acid plating bath and to a method for the electrolytic deposition of satin nickel deposits. Electrolytes for obtaining matte nickel deposits, by contrast, do not form part of this invention.
In nickel electroplating, one generally tries to achieve a bright, level deposit. It has also soon been found out that silk matte deposits have an aesthetic appearance while preventing disturbing blinding effects. Combined with semi-bright nickel and with a chromium layer, such type layers provide the same protection from corrosion as a bright nickel layer. These satin nickel layers are widely used in the automotive industry, in precision mechanics, in the sanitary industry and eventually even in the furniture industry.
Hereto before, the satin effect could be produced using various methods. At first, the satin effect was obtained using mechanical methods with the bottom layer being matted by sandblasting. Later, insoluble substances of a certain fineness such as glass, French chalk, barium sulfate, graphite, kaolin or similar substances were added to the nickel electrolyte. Whereas the first method involved a considerable expense and did not fit in the electroplating process, the satin effect obtained using the insoluble substances was rougher than silk matte and had an irregular surface.
Organic substances that are difficult to dissolve, comprising in part stabilizing wetting agents did not show any lasting success:
DE-OS 1 621 085 discloses an acid nickel plating bath to provide satin nickel deposits that, in addition to primary brighteners, contains a concentration of such type substituted or unsubstituted adducts of ethylene oxide or propylene oxide or ethylene oxide/propylene oxide which, at a temperature of 40-75° C., form a fine emulsion in the electrolyte bath with said concentration ranging from 5 to 100 mg/l.
Further, DE 25 22 130 B1 describes an acid, aqueous nickel plating bath, nickel/cobalt plating bath or nickel/iron plating bath for depositing silk matte layers that contains, in addition to the primary and/or secondary brighteners, emulsified liquid polysiloxane polyoxyalkylene block copolymers.
Moreover, in Patent Abstract of Japan, the document JP 56152988A discloses a nickel bath for depositing satin coatings that contains, in addition to saccharine as a brightener and to polyoxyethylene-polyoxypropylene block copolymers, wetting agents selected from the group of the alkylaryl sulfonates and of esters of sulfosuccinic acid. In this case as well it was established that a satin nickel layer can only be obtained for a short period of time after the bath has been prepared. After that, the coatings obtained are rough and unsightly.
DE 21 34 457 C2 furthermore discloses an aqueous electroplating bath for depositing bright nickel or nickel/cobalt layers. According to some examples, an ester of sulfosuccinic acid is, among others, added to baths already containing saccharine as a secondary auxiliary brightener. These baths are not used to produce satin layers.
A method that has gained much more acceptance makes use of adducts of polyalkylene oxide, mostly adducts of ethylene oxide/propylene oxide, with water or aliphatic alcohols, that dissolve completely in the cold nickel electrolyte but are insoluble at an operating temperature of 50-60° C. (DE-OS 1 621 087). It is known that, upon exceeding the cloud point temperature, the non ionogenic surface active agents precipitate by getting rid of their hydrate shell. These precipitating drops selectively disturb the deposition of nickel without being substantially incorporated into the nickel. The disadvantage of this method is the high expense of energy for heating and cooling as well as for pumping. The maximum volume of the bath is also restricted since, as it reaches about 8,000 liter, the expense for heating, cooling and pumping increases dramatically. Moreover, agglomerates, which produce black pits, often form after a short period of time.
In view of the shortcomings described, a method is gaining increasing acceptance in which quaternary ammonium compounds are utilized in the bath. DE 23 27 881 A1 describes a method of producing matte nickel deposits or nickel/cobalt deposits by which the matt deposits are obtained by incorporating foreign substances. The foreign substances are achieved by combining cationic active or amphoteric substances with organic anions. Possible cationic active or amphoteric substances are quaternary ammonium compounds, imidazoline derivatives, esters of alkanolamines and surfactants based on amino carboxylic acid. Together with the anionic primary brighteners contained in the nickel electrolyte, the cationic active substances form ion pairs that are difficult to dissolve and that produce a satin effect by disturbing the nickel deposition process. Unfortunately, this method also has disadvantages:
Within approximately 3-5 hours the precipitating, difficult to dissolve ion pair crystallites increase in size and produce an increasingly coarse nickel surface or even clearly visible coarse single nickel crystals (“diamonds”) that are very detrimental to the appearance of the nickel surface. Therefore, the production must be disrupted after 8 hours at the latest to completely filter and clean the electrolyte using filtering means such as a cellulose filter, kieselguhr or even activated carbon. This disruption in production is very disturbing and very costly, more specifically if an automatic machine is being used. Moreover, a film that may be wiped off often forms after chromium plating for 10 minutes and longer (“silver layer”).
Some attempts have been made to overcome this shortcoming. One solution consisted for example in combining the two methods and in adding organic, aromatic sulfinic acids to the bath intended to produce satin nickel deposits. Such a bath composition is described in DE 37 36 171 A1. In this case, no optically uniform deposits are obtained without cooling and heating.
The use of a concentration of highly effective non ionogenic wetting agents (polyethylene glycol monomethyl ether) so small that the bath lacks any visible cloudiness is not successful either. DE 195 40 011 A1 indicates another method for the electrolytic deposition of nickel deposits with no blinding effect that makes use of a nickel bath containing inter alia primary brighteners, organic sulfinic acids and wetting agents. The bath further contains a concentration of substituted and/or unsubstituted adducts of ethylene oxide or of propylene oxide or of ethylene oxide/propylene oxide so small that the bath lacks any visible cloudiness at the operating temperature of the bath. The use of the indicated concentration of non ionogenic wetting agents is not successful because their efficiency decreases very soon and the appearance of the deposit quickly changes.
All of the methods described can only be operated for a few hours. An improvement was achieved by using esters of sulfosuccinic acid together with ammonium compounds (DE 100 25 552 C1). The high amount of nickel ions in excess of 105 g/l required and the sensitivity to foreign wetting agents (that have been dragged in) are disadvantageous, though. Furthermore, the bath, which needs cleaning, can only be successfully cleaned with active carbon, which is quite inconvenient to handle since the filter can only be used once and the filter residue has to be disposed of after each cleaning. On the other side, problems arise during chromium plating because of the formation of a film that may be wiped away (“silver layer”).